SP484, U.S. #1818-20, Nondenominated "B" Booklet Stamps, USPS Souvenir Page. First Day of Issue Mar. 15, 1981, San Francisco, CA 94101. Scan is an example. Never folded, shipped flat in a sheet protector.
USPS Souvenir Pages are -Official First Day of Issue Cancellations, which are a mint stamp or stamps affixed to an 8"x10.5" color page, and include technical and historical information about the stamp or stamps and the designer. The pages were issued flat and unfolded.